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Session Length
~17 min
Adaptive Checks
15 questions
Transfer Probes
8
Cryptography is the science and practice of securing communication and data through the use of codes, ciphers, and mathematical algorithms so that only intended recipients can access the information. Rooted in ancient traditions of secret writing, modern cryptography has evolved into a rigorous mathematical discipline that underpins the security of digital communications, financial transactions, and virtually every aspect of online life. At its core, cryptography transforms readable plaintext into unreadable ciphertext using encryption algorithms and keys, ensuring confidentiality, integrity, authentication, and non-repudiation.
The field underwent a revolutionary transformation in the 1970s with the introduction of public-key cryptography by Whitfield Diffie, Martin Hellman, and Ralph Merkle. Unlike symmetric-key systems where both parties must share a secret key, public-key cryptography uses mathematically related key pairs, allowing secure communication between parties who have never met. This breakthrough, along with the RSA algorithm developed by Rivest, Shamir, and Adleman, made secure electronic commerce and digital signatures possible. The development of the Advanced Encryption Standard (AES) further solidified the role of standardized cryptographic algorithms in protecting sensitive data worldwide.
Today, cryptography faces both extraordinary opportunities and challenges. Quantum computing threatens to break many widely used cryptographic systems, spurring urgent research into post-quantum cryptography. Meanwhile, cryptographic innovations like homomorphic encryption, zero-knowledge proofs, and blockchain technology are enabling entirely new paradigms for privacy, trust, and decentralized computation. From securing personal messages and banking transactions to protecting national security communications, cryptography remains one of the most critical and rapidly evolving fields at the intersection of mathematics, computer science, and information security.
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An encryption scheme where the same secret key is used for both encryption and decryption. Both the sender and receiver must possess the identical key, which must be kept secret from all other parties.
Example: AES-256 is used to encrypt files on your hard drive. The same password-derived key that encrypts the data is needed to decrypt it, so anyone who obtains the key can read the files.
A cryptographic system that uses a pair of mathematically related keys: a public key that can be freely shared for encryption, and a private key kept secret for decryption. It solves the key distribution problem inherent in symmetric cryptography.
Example: When you visit an HTTPS website, your browser uses the server's public key to encrypt a session key. Only the server's private key can decrypt it, establishing a secure connection without prior key exchange.
One-way mathematical functions that take an input of any size and produce a fixed-size output (digest) that is computationally infeasible to reverse. Even a tiny change in input produces a completely different hash, a property called the avalanche effect.
Example: When you create a password on a website, the site stores a SHA-256 hash of your password rather than the password itself. When you log in, it hashes your input and compares the hashes.
A cryptographic mechanism that allows the signer to prove the authenticity and integrity of a message. The sender signs with their private key, and anyone can verify the signature using the sender's public key, providing authentication and non-repudiation.
Example: Software developers digitally sign their applications so your operating system can verify the software hasn't been tampered with and truly comes from the claimed developer.
Methods that allow two parties to establish a shared secret key over an insecure channel without any prior shared secret. The Diffie-Hellman protocol was the first practical solution to this fundamental problem.
Example: When your browser connects to a website, the Diffie-Hellman key exchange allows both parties to agree on a shared encryption key, even though an eavesdropper can see all the exchanged messages.
Block ciphers encrypt fixed-size blocks of plaintext (e.g., 128 bits for AES), while stream ciphers encrypt data one bit or byte at a time using a pseudorandom keystream. Each has different performance characteristics and use cases.
Example: AES (a block cipher) is commonly used for encrypting files and disk storage, while ChaCha20 (a stream cipher) is favored for encrypting real-time communications like video calls due to its speed on mobile devices.
The study of methods for obtaining the meaning of encrypted information without access to the secret key. It encompasses techniques ranging from brute-force attacks to sophisticated mathematical analysis exploiting weaknesses in cryptographic algorithms.
Example: The Enigma machine used by Germany in World War II was broken by Allied cryptanalysts at Bletchley Park, including Alan Turing, by exploiting structural weaknesses and operator errors.
A framework of policies, hardware, software, and procedures used to create, manage, distribute, store, and revoke digital certificates. PKI establishes a chain of trust that binds public keys to the identities of their owners.
Example: When your browser shows a padlock icon on a website, it has verified the site's SSL/TLS certificate through a chain of trust back to a trusted Certificate Authority like Let's Encrypt or DigiCert.
More terms are available in the glossary.
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